(a) Field of the Invention
The present invention relates to a diaphragm for use in electro-acoustic transducers such as loudspeakers, headphones, microphones and the like, and more particularly it pertains to a diaphragm having a honeycomb structure as its core member.
(b) Description of the Prior Art
As well known to those skilled in the art of audio devices, diaphragms for use in such electro-acoustic transducers as a loudspeaker are required to make a piston motion through a wide frequency range, as well as to have an internal loss of an appropriate magnitude. The piston motion of a diaphragm is impaired by the development of partial vibrations in the diaphragm itself. These partial vibrations are roughly classified into the following two types, one of which is axisymmetric partial vibration which constitutes a factor of determining the applicable frequency range of a diaphragm in its designing, and the other is non-axisymmetric partial vibration which serves as a factor to increase the distortion. The axisymmetric partial vibration may be suppressed by, for example, increasing the thickness of the diaphragm and increasing the ratio of Young's modulus E to density .rho., i.e. Young's modulus-to-weight ratio E/.rho.. Also, the non-axisymmetric vibration may be suppressed by, for example, arranging the diaphragm so as to be uniform in both structure and material with which it is made, and to have a substantial thickness, and also to have a substantially large Young's modulus-to-weight ratio E/.rho.. On the other hand, the internal loss dominates the magnitude of the peak level of the high frequency range resonance curve in SPL (sound pressure level)/frequency response. In case the difference between the peak and the bottom of the high frequency range resonance curve is great, the sound produced will be perceived as unpleasant in its quality to listeners. It is known to be possible to avoid the offensiveness, to the ears, of a sound reproduced, by setting the internal loss .eta. (dissipation factor tan .delta.) at an appropriately large value, thereby reducing the peak level of the high frequency range resonance curve to a low level.
Accordingly, a diaphragm is required to have a substantially large thickness and a large Young's modulus-to-weight ratio E/.rho. to provide both light-weight and increased rigidity features, as well as required to have a piston motion through a wide range of frequency, and further to have an appropriately large internal loss.
Recently, there have been put to use acoustic diaphragms for use in such audio devices as loudspeakers, which employ a honeycomb core member made with a metal sheet or a high molucular sheet material and provided with a skin member adhering to a surface of this core member. Since the diaphragm of this type has a honeycomb core structure, the diaphragm can be manufactured to be light in weight and high in rigidity. Also, the apparent density .rho. of the diaphragm as a whole is low, so that the Young's modulus-to-weight ratio, i.e. the ratio E/.rho. between Young's modulus E and density .rho., is large, whereby it is possible to obtain a good sound reproduction characteristic as represented by piston motion through a wide range of frequency and by a reduced distortion for a wide range of frequency. Besides, such honeycomb core type diaphragm laminated with the skin is known to have an advantageous structure which allows to set its internal loss at an appropriately large value due to attenuation of vibration attributable to the laminated structure, thereby substantially suppressing the development of resonance in the high frequency range all allowing a flat frequency characteristic to be obtained.
In spite of such superior structure of the known diaphragm of this type as stated above, it is the actual state of such diaphragm that no sufficiently satisfactory diaphragm structure has been provided. More particularly, in the known diaphragms of this type, the properties of the skin member per se for use in the laminated structure give a great influence on the general characteristics of the diaphragm as a whole. Those skin members which have been employed in known diaphragms invariably have strong points and weak points, and thus they are still far from satisfying all of the requirements expected of a diaphragm. For example, there are skin members which have been used widely for diaphragms of this laminated structure type and are made with a paper sheet formed with fibers of wood pulp or with a metal foil such as aluminum foil. However, while a paper skin member made of wood fibers has a large internal loss .eta. (dissipation factor tan .delta.), thus making difficult the tendency to develop resonance in the high frequency range and also providing a flat frequency characteristic, the paper skin member has a low Young's modulus E, resulting in a low efficiency in the reproduction of sound, and has a low Young's modulus-to-weight ratio E/.rho. in the completed diaphragm, causing a narrow range of piston motion, and thus there cannot be obtained such a good sound reproduction characteristic as represented by reduced distortion in the high frequency range and by a superior transition characteristic. On the other hand, skin members using a metal foil have the advantages that they have a relatively high Young's modulus E, allowing the Young's modulus-to-weight ratio E/.rho. to be made large and thus providing a sound reproduction characteristic as represented by reduced distortion through a wide frequency range and superior in transition characteristic. However, the metal foil skin member, while its having a honeycomb core member which should allow the internal loss to be made large, has the property that its internal loss is markedly small, and thus there is the actual problem that a sharp resonance develops in the high frequency range so that the reproduced tone in the high frequency range tends to become irritating to the ears of listeners.
Hereunder is shown in Table 1 the result of comparison between skin members made with metal foil and skin members made with paper.
TABLE 1 ______________________________________ Young's modulus-to- Material Young's weight ratio Internal of skin modulus(E) Density(.rho.) (E/.rho.) loss member (10.sup.6 N/m.sup.2) (10.sup.3 kg/m.sup.3) (10.sup.6 m.sup.2 /sec.sup.2) (tan .delta.) ______________________________________ Titanium 110,000 4.5 24.01 0.002 Aluminum 70,000 2.7 26.01 0.002 Paper 2,000 0.5 4.00 0.05 ______________________________________
Furthermore, there have been proposed skin members formed with a carbon fiber-reinforced plastic material. While a carbon fiber-reinforced plastic material has a large Young's modulus-to-weight ratio E/.rho., its strength-to-weight ratio cannot be necessarily termed as being sufficient, and in addition there is the disadvantage that its manufacturing cost is high.